Microbiology · Year 2 · from Microbiology
Case 2: Cholera
Presentation
A 45-year-old aid worker recently returned from a humanitarian mission in a region experiencing a cholera outbreak presents to the emergency department with profuse watery diarrhea that began 18 hours ago. She reports more than 20 episodes of stool that looks like "rice water" - clear liquid with flecks of mucus. She also has had multiple episodes of vomiting. On examination, she appears severely dehydrated with sunken eyes, dry mucous membranes, poor skin turgor, and weak pulses. Vital signs: temperature 36.2°C, heart rate 130 bpm, blood pressure 70/40 mmHg. She is alert but weak and complains of muscle cramps.
Clinical Image
Characteristic "rice-water" stool of cholera - clear fluid with mucus flecks but no blood or inflammatory cells, reflecting the secretory nature of the diarrhea.
Image Source: Lecture image - Vibrio cholerae infection
Questions
- What is the causative organism and what toxin is responsible for the clinical syndrome?
- Explain the molecular mechanism by which the toxin causes profuse diarrhea.
- What is the cornerstone of treatment, and why is it so effective?
- What role do antibiotics play in management?
Answers
- Organism and toxin: The causative organism is Vibrio cholerae (serogroup O1 or O139), and the clinical syndrome is caused by cholera toxin. This is an A-B toxin with a pentameric B subunit that binds to GM1 ganglioside on intestinal epithelial cells and an A subunit that exerts the toxic effect. V. cholerae is a curved gram-negative rod with a single polar flagellum, found in contaminated water and shellfish in endemic regions.
- Molecular mechanism: Cholera toxin's A subunit ADP-ribosylates the stimulatory G protein (Gsα), permanently locking it in the active state. This leads to sustained activation of adenylyl cyclase, causing persistently elevated intracellular cyclic AMP (cAMP). Elevated cAMP activates the CFTR chloride channel, causing massive secretion of chloride ions into the intestinal lumen. Sodium and water follow osmotically, producing the profuse, watery "rice-water" diarrhea that can exceed 20 liters per day in severe cases.
- Treatment cornerstone: Aggressive fluid and electrolyte replacement is the cornerstone of treatment. Oral rehydration solution (ORS) is remarkably effective and can be lifesaving even in severe cases because the sodium-glucose cotransporter remains functional and can absorb water even when chloride secretion is maximized. Intravenous fluids (Ringer's lactate) are needed for severe dehydration or patients unable to tolerate oral intake. The mortality of cholera drops from over 50% to less than 1% with adequate rehydration.
- Role of antibiotics: Antibiotics are adjunctive therapy that shortens the duration and volume of diarrhea, typically by 50%, and reduces bacterial shedding. First-line agents include doxycycline (single dose) or azithromycin. However, antibiotics are secondary to rehydration and should not delay or replace aggressive fluid therapy. Antibiotic resistance in V. cholerae is emerging in some regions, which may limit effectiveness.